In an OFDMA mobile communication system, frequency resources are assigned to multiple access users in resource blocks each containing a group of consecutive subcarriers. Currently, the standardizing institution is discussing a frequency bandwidth for each resource block around the core frequency of 375 kHz. Specifically, the effect of fading occurring in a mobile communication environment is reduced by optimum resource assignment in adoption of multiuser scheduling to improve the maximum throughput of the system. According to this, a bandwidth for each resource block is determined so that the effect of fading becomes virtually flat in the band, and, therefore, a degree of fading varies greatly between different resource blocks.
A study of precoding MIMO has been in progress for the purpose of a further improvement in throughput and in communication quality. In precoding MIMO, the number of data streams and a transmission beam in MIMO are changed depending on a communication environment to be able to optimize both beam forming gain and throughput (see, e.g., Published Japanese translation of a PCT application No. 2005-522086 (hereinafter “Patent Document 1”)).
In a certain wide-band radio connection system using a multiple antenna in MIMO, precoding information, etc., for the multiple antenna is transmitted in the form of a message through a downlink to give an instruction to a terminal unit (see, e.g., Japanese Laid-Open Patent Application Publication No. 2006-141013 (hereinafter “Patent Document 2”)).
A concept of a resource block in OFDMA will then be described. FIG. 21 depicts resource blocks in OFDMA. For example, as depicted in FIG. 21, subcarriers f1 to fN are divided into a plurality of resource blocks (RB) (#1 to #n, and for example, n=12) in given unit frequency (375 kHz) (total frequency range 5 MHz=375 kHz×12). Conventionally, a mobile station at the reception side selects one subcarrier from each of n resource blocks to select n optimum subcarriers (beams) in total and feeds the selected subcarriers back to a base station.
FIG. 22 is a block diagram of a conventional radio communication system. A base station 10 and a mobile station 50 are each equipped with a radio communication apparatus. A precoder 11 of the base station 10 receives input data streams, selects a beam according to a code book 12 when carrying out beam selection, and outputs a plurality of pieces of data (four pieces of data in FIG. 22). To each piece of output data from the precoder 11, a pilot signal of an orthogonal component (orthogonal pilot) is added by an adding unit 13. A transmitting unit 14 modulates the data to output the modulated data via a plurality of antennas 15. A control unit 16 specifies a value to be read out of the code book 12 at the time of beam selection, based on input data from the mobile station 50, and gives the specified vale to the precoder 11.
The mobile station 50 receives data via a plurality of antennas 51 (two antennas in FIG. 22). The received data is input to a channel estimating unit 52, which estimates a channel using an orthogonal pilot component contained in the received transmitted data. A demodulator 53 demodulates stream data to output demodulated data. The mobile station 50 has a code book 54 whose contents are identical to those of the code book of the base station 10. A beam measuring unit 55 determines an ID (weight) for precoding, based on a channel estimation value estimated by the channel estimating unit 52 and the contents of the code book 54. At this time, the beam measuring unit 55 measures a beam having the maximum SINR (SIR), and determines a weight for a beam for each of n resource blocks. A selecting unit 56 ranks beams for each resource block to select an optimum beam, and transmits the selected beam as a feedback signal to the base station 10. The beam measuring unit 55 and the selecting unit 56 make up a control means in the mobile station 50.
FIG. 23 depicts a conventional method of determining a weight. The beam measuring unit 55 of FIG. 22 determines one weight for each of n resource blocks (RB), thus determining n weights in total. The selecting unit 56 applies determined n weights to n resource blocks (RB), respectively, to transmit weighted beams to the base station 10.
The technique for MIMO precoding described in Patent Document 1, however, does not disclose the use of resource blocks and determination of a weight in OFDMA. By this technique, therefore, optimizing precoding through MIMO at the base station 10 is impossible, whereby transmission quality in communication may not be improved. Likewise, the technique described in Patent Document 2 does not disclose a configuration related to determination of a weight for precoding. By this technique, therefore, optimizing precoding through MIMO at the base station 10 is impossible, whereby transmission quality in communication may not be improved.
MIMO precoding desires not only improved weight determination but also improved transmission quality in transmission of a feedback signal containing a determined weight from the mobile station 50 to the base station 10. In a configuration according to a conventional technique (depicted in FIG. 22, etc.), feedback information carries a weight for each resource block in transmission. Because of this, the base station 10 is incapable of accurately receiving the feedback information due to a change in the reception environment of a transmission path, etc., which makes impossible an improvement in the transmission quality of the feedback information. As a result, optimizing precoding through MIMO may not be achieved, whereby transmission quality in communication may not be improved.